Quote:
Originally Posted by anfo
How so? Whats the function of it, and what does it correct?
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Hi Aaron,
Equatorial mounts are essentially the same as Alt/Azimuth mounts, such
as a Dob, except in an equatorial mount the Azimuthal axis is 'pushed over'
in elevation so that it is, for us in the Southern Hemisphere, then pointing
toward the South Celestial Pole.
One advantage of this is that the scope then only need track primarily in one axis,
namely the Az axis which is now called the RA axis on an equatorial mount,
in order to follow a star.
The other primary axis is known as the Alt axis on an Alt/Az mount and it
then becomes what is known as the Dec axis on an equatorial mount.
As the star tracks across the sky, the scope then need not move in Dec except
to compensate for small amounts of apparent motion that are caused by
atmospheric refraction.
If one uses an Alt/Az scope to track the stars, its motors need to be driving
both the Alt and Az axes simultaneously and what's more, the rate at
which those two axes have to be driven at is constantly changing. As a second
side-effect of tracking in Alt/Az mode, if you were to observe a field of stars through
the eyepiece over time as the mount tracks, they will appear to rotate around
an axis through the middle of the field of view. If you were to attach a camera and
take a time-lapse, rather than steady pinpoints of light at the same location, the
stars would appear to rotate within the field of view. This is known as
field rotation.
Some mounts add a third motor to where the eyepiece is to rotate the camera
as the mount tracks. Such a device is called a field derotator.
On large Dobs we have customers who take short exposure photographs of
DSO's and then stack multiple shots in software and use software field de-rotation.
The key then is to keep the exposure times short, which is at odds with the
desire to keep the exposure times long but beneath the noise levels to capture
the maximum amount of information. Nevertheless, some of these customers
have produced fabulous results with, for example, servo motor driven telescopes with
apertures in the 20" F/4.5 class.
Though most of the world's large professional scopes are driven Alt/Az and
use field de-rotators, it is not common to see field de-rotators in use when
you walk around the field of a star party. What are more common as
an imaging platform are then true equatorial mounts.
Because of the effects of atmospheric refraction, even equatorial scopes have
a limitation in that if the exposure times are long enough, some field rotation will
still occur. Some exotic telescopes, used for photographic sky surveys,
use an equatorial mount but then drive the scope in elevation (altitude) as well
to compensate for field rotation phenomena. For most enthusiasts, exposure
times are sufficiently short that any residual field rotation on an equatorial
mount does not impact the final image by any discernible amount.
Some Dob owners purchase or construct a device called an equatorial platform or
equatorial table. This is a motor driven 'wedge' that the Dob then sits on and it converts it
to an equatorial mount.
Skywatcher Auto-tracking dobs
:camer a:
